Did dry ice chunks on Mars gouge strange tracks into Red Planet?

Images from the Mars Reconnaissance Orbiter show some of the planet's long "linear gullies." Scientists have concluded they were carved by dry ice.

Images from the Mars Reconnaissance Orbiter show some of the planet's long "linear gullies." Scientists have concluded they were carved by dry ice. (NASA)

Amina Khan

Add this to your list of winter sports: sledding down Mars' sand dunes on a sled of dry ice. Scientists say they have finally determined that mysterious, long gouges down the Red Planet’s slopes were caused by large chunks of frozen carbon dioxide.

The findings, described in the journal Icarus, shed light on strange features -- so-called linear gullies -- that are unlike any observed on Earth.

Picked up by the Mars Global Surveyor, launched in 1996, the strange, long grooves puzzled planetary scientists for years. Unlike flows of water on Earth, which naturally create a sort of fan shape as debris travels down a hill, these grooves were deep and straight and were punctuated by a pit at the end.

"There's been a variety of other hypotheses discussed for these linear gullies, and I didn’t know which was right," said University of Arizona planetary scientist Alfred McEwen, a study co-author and lead scientist for a camera on the Mars Reconnaissance Observatory satellite known as HiRISE, which has allowed researchers to take a closer look at the linear gullies in recent years.

Mars gets far colder than Earth during its winter times, cold enough for carbon dioxide to form frost in its chillier regions. As spring warmed these icy regions, the researchers noticed that these strange gouges seemed to reappear.

To test out a theory, study co-author Candice Hansen, HiRISE’s deputy scientist at the Planetary Science Institute in Tucson, bought slabs of dry ice from the supermarket and slid them down slopes in deserts in Utah and California. Sure enough, the same gouges seen on Mars emerged.

The scientists believe that the gullies are caused by large chunks of frozen carbon dioxide -- some a couple meters in diameter -- breaking off as Mars warms up during springtime. These blocks of dry ice plow down the slopes and come to rest, leaving an empty pit after they’ve completely sublimated, or turned straight from solid into gas. Gaseous carbon dioxide coming off of the ice slabs acts as a lubricant underneath the block and pushes material to the sides, McEwen said, giving the gouge high walls and making them appear even deeper.

There are still a number of unanswered questions, McEwen said, including how long the slabs of carbon dioxide stay solid once they come to a full and complete stop.

"We're starting to really understand Mars today, and that gives you the baseline for understanding Mars in the past," McEwen said.